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IS-IS An introduction to IGP routing protocols Hagai Kahana

IS-IS An introduction to IGP routing protocols Hagai Kahana. 57.13.29.2. 156.10.24.1. 16.55.42.10. AS 22. AS 33. AS 11. AS 11. iBGP eBGP. Interior Gateway Protocol (IGP) – intended to use within an AS. IGP routing protocols. Distance-vector routing protocol

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IS-IS An introduction to IGP routing protocols Hagai Kahana

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  1. IS-IS An introduction to IGP routing protocols Hagai Kahana

  2. 57.13.29.2 156.10.24.1 16.55.42.10 Compass-EOS Confidential

  3. AS 22 AS 33 AS 11 AS 11 iBGP eBGP • Interior Gateway Protocol (IGP) – intended to use within an AS Compass-EOS Confidential

  4. IGP routing protocols • Distance-vector routing protocol • RIP - Routing Information protocol – used for small networks • EIGRP- Enhanced Interior Gateway Routing Protocol – Cisco proprietary routing protocol – used for medium size networks • Link-state routing protocol • OSPF - Open Shortest Path First – Design for IPv4 networks, a similar to IS-IS and more common in enterprise networks • IS-IS Compass-EOS Confidential

  5. IS-IS introduction • Was first published as ISO 10589 in 1987 • IETF republished the protocol in RFC 1142 for the Internet community and extended for IP routing as RFC 1195 in 1990 • Runs natively on Layer 2 of the OSI model (Ethernet/PPP) Compass-EOS Confidential

  6. Link-state protocol – High-level description • Neighbor discovery – • How two link state router discover each other and agree to exchange routing information Compass-EOS Confidential

  7. Link-state protocol – High-level description • Flooding – • How routing information is forward reliably to all routers in a network Compass-EOS Confidential

  8. Link-state protocol – High-level description • Link state database – • How routing information is stored, synchronized and kept up to date Compass-EOS Confidential

  9. Link-state protocol – High-level description • SPF (short path first) calculation – • How we use the information to calculate the routes Compass-EOS Confidential

  10. IS-IS introduction • Neighbor discovery. • Flooding. • Link state database Synchronization. • SPF (short path first) calculation. Compass-EOS Confidential

  11. IS-IS Addressing System ID – Unique identifier, can be set by using the IP loopback address Area ID Compass-EOS Confidential

  12. TLV • Type-Length-Value • Why length if we have type? • Flexibility • Max of 256 bytes • Contains sub-TLV • Major advantage over OSPF TYPE LENGTH VALUE . . . Length in Octets 1 1 Length of value Compass-EOS Confidential

  13. TLV • TLV 10 – authentication TLV • Added to each PDU transmitted • Packet discarded when not matched • Key - password for the PDU type • Text - IS-IS PDU to be authenticated Authentication value Type (10 ) Length (Variable) Authentication type Compass-EOS Confidential

  14. IS-IS Common Header ISIS Protocol Discriminator Length Indicator Version ID Extension ID Length PDU Type Version Reserved Packet Data … Maximum Area Addresses Compass-EOS Confidential

  15. Neighbor discovery

  16. Neighbor discovery IIH message • Once you enable IS-IS on an interface, the routing protocol will automatically find out if there are IS-IS router at the other end. • If one found a verification of version and two way interaction on the link is been done – called handshaking. • Handshaking is done with hello message called IIH (Intermediate System to Intermediate System Hello) message. • All messages sent to 2 multicast MAC addresses 0180:c200:0014 and 0180:c200:0015 on Broadcast media no unicast messages.

  17. Neighbor discovery -cont ISIS Protocol Discriminator Length Indicator Version ID Extension ID Length PDU Type Version Reserved Maximum Area Addresses TLV Fields Two types of connection broadcast LAN and P2P. Different IIH for each connection type P2P IIH are for both levels (bandwidth consideration)

  18. Handshaking – 3 ways Router 1 Router 2 IIH Router 1 IIH Router 2 “I see Router 2” + TLVs “I see Router 1” + TLVs IIH Router 1 TLV s Adjacency UP Adjacency UP Router 1 sends router 2 IIH. Router 2 Gets IIH from 1 and immediately send an IIH back which says “I have seen 1”. Route 1 receive IIH from route 2, set Adj as up and immediately send an IIH back which says “I have seen 2”. Router 2 receive second IIH from 1 and set Adj to up Compass-EOS Confidential

  19. IIH – LAN Level 1 and Level 2 ISIS Protocol Discriminator Reserved circuit type Length Indicator Source IS Version ID Extension Holding time PDU Length ID Length R Priority PDU Type Version DIS TLV Reserved Maximum Area Addresses TLV Fields Compass-EOS Confidential

  20. IIH – IS-IS Hello Packet • Hello Interval – time between each IIH packets • Hold-time - maximum time lapse allowed between receipts of two consecutive hello packets received • If hold-time expire an adjacency is declared “dead” • Can be modified on the fly (unlike OSPF) Compass-EOS Confidential

  21. IIH – IS-IS Hello Packet Hello Packet 40sec hold time 40 Hello Packet 30sec hold time Hello Packet 30sec hold time 30 20 10 • IIH sent in these cases: • Expiration of the hello interval • Any change in network conditions • Election to or resignation from LAN DIS position Compass-EOS Confidential

  22. Flooding

  23. LSP Link State Protocol data unit • Link-state routing distribute their topological view beyond their immediate neighbor (unlike vector distance). • A single router create a LSP (Link state PDU -> Link State Protocol Data Unit) that contain the reachability and immediate connected neighbored (imagine it as a single part of a puzzle). Compass-EOS Confidential

  24. LSP Link State Protocol data unit • When a router receive updated LSP from all the routers in the network it has a complete map view of the puzzle and can accurately compute paths in the network. • The process of sending and updating topology information is called flooding. • The LSP is stored in LSP Data base, a different DB exist for each level • LSP contain other information such as IP reachability, check sum and even the routers name Compass-EOS Confidential

  25. LSP Revision control Which LSP is the most recent one? • Sequence number – version of the LSP. • Every new LSP generated the router increase the sequence number by one. • The Sequence number is stored in a 32-bit identifier. If a new LSP is sent every 5 second we have 681 years till the end of the address space Compass-EOS Confidential

  26. LSP Revision contol • LSP Lifetime default is 20 minutes (a 16 bit field) • Periodic Refreshes • LSP-ID – a unique identifier of the router consist of the System ID, pseudo-node ID and fragment ID (8B total) • Sequence number – version of the LSP. • Checksum – check if info corrupt on the way IMI#showisis database Area ipi: IS-IS Level-2 Link State Database: LSPID LSP Seq Num LSP Checksum LSP Holdtime ATT/P/OL 0000.0000.0001.00-00* 0x0000035F 0x24D8 506 0/0/0 Compass-EOS Confidential

  27. LSP Link State Protocol data unit Intradomain Routing Proto Descriminator Lenth Indicator Version/Protocol ID Extension ID Length R R R PDU Type Version Reserved Maximum Area Addresses PDU Length Remaining Lifetime LSP ID Sequence Number Checksum P ATT LSPDBOL IS Type TLVFIELDS LSPs are acknowledged by the receiving router A router must not originate more than one LSP every 5 seconds

  28. LSP Link State Protocol data unit • Purge LSP - negative LSP update • DIS election • Expiration of LSP • Duplicate System IDs Compass-EOS Confidential

  29. Link state database Synchronization.

  30. Pseudo-nodes and Designated Routers • A LAN Topology is equivalent to full mash P2P connection. • Effect of an additional router attached to the LAN Compass-EOS Confidential

  31. Pseudo-nodes and Designated Routers LSP 1111.1111.1111.01-00 Nbr 1111.1111.1111.00-00 Nbr 2222.2222.2222.00-00 Nbr 3333.3333.3333.00-00 Nbr 4444.4444.444.00-00 Nbr 5555.5555.5555.00-00 Nbr 6666.6666.6666.00-00 LSP 1111.1111.1111.00-00 LSP 2222.2222.2222.00-00 Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00 LSP 4444.4444.4444.00-00 LSP 6666.6666.6666.00-00 LSP 3333.3333.3333.00-00 LSP 5555.5555.5555.00-00 Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00 Nbr 1111.1111.1111.01-00 Pseudo-node (PSN) is a virtual node, whose role is played by an elected DIS for the LAN. Compass-EOS Confidential

  32. Pseudo-nodes and Designated Routers • DIS election is preemptive, based on interface priority with the highest SNPA (MAC address) breaking ties • Separate DISs are elected for Level 1 and 2 routing • DIS transmits IIH three times faster other routers, for quick detection of failure and immediate replacement. Compass-EOS Confidential

  33. Synchronizing Databases in LANs Router 2 Router 1 CSNP PSNP LSP Router XXXX.00-00 Life time 1678 Seq 0x05 Checksum 0xABCD Router XXXX.00-00 Life time 1678 Seq 0x05 Checksum 0xABCD Router …. …. Router XXXX.00-00 Life time 0 Seq 0x00 Checksum 0xABCD Compass-EOS Confidential DIS sends periodically a directory of its link-state database Compass-EOS Confidential

  34. SPF (short path first) calculation.

  35. SPF • There are three type of SPF calulation • Full SPF – Find the shortest path tree on the topology – also called first pass. • Partial SPF – Extract prefix and metric information from link state database – also called second pass. • Incremental SPF – optimization on the full SPF calculation Compass-EOS Confidential

  36. SPF – Hold down • A trigger for SPF calculate does not invoke the calculation immediately. • A change in the network invoke several LSP flooding. • An SPF cause a burst of CPU usage without hold down the router will choke under the SPF calculation stress. Compass-EOS Confidential

  37. Thank you Compass-EOS Confidential

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