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IS-IS WG - IETF 70

Summary Route with Detailed Reachability George Swallow, Clarence Filsfils, Stefano Previdi swallow@cisco.com cfilsfil@cisco.com sprevidi@cisco.com. IS-IS WG - IETF 70. Agenda. Motivation Solution Sketch Open Issues Encoding technique Inconsistent advertisements. Motivation.

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IS-IS WG - IETF 70

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  1. Summary Route with Detailed ReachabilityGeorge Swallow, Clarence Filsfils, Stefano Previdiswallow@cisco.com cfilsfil@cisco.com sprevidi@cisco.com IS-IS WG - IETF 70

  2. Agenda Motivation Solution Sketch Open Issues Encoding technique Inconsistent advertisements

  3. Motivation • Scalability and convergence • IGP convergence • BGP Convergence • Example: L3VPN over L2TPv3 • Other Uses • PIM • RSVP-TE mesh

  4. Some Brief Notes on IGP Convergence • Testbed • CRS1 • 2500 ISIS prefixes • Tier 1 ISP Topology • Time measured by traffic loss Prefix Number • Time for ISIS LSP generation, SPF recalculation is very quick • Substantial time is required for update of structures on linecards for individual prefixes • Time shown is IP, for MPLS LFIB needs updating too

  5. L3VPN over L2TPv3 Area 2 10.10.2.2 PE2 ABR1 ABR3 ABR2 PE1 PE3 10.10.0.1 10.10.1.1 Area 0 Area 3 Area 1 10.10.0.2 10.10.0.3 10.10.2.3 • VPN packets are encapsulated in L2TPv3 • For many VPNs, multiple next-hops are carried in BGP using a Route Distinguisher (RD) • Switch to new route occurs on BGP withdrawal or indication from ISIS that the next-hop is not reachable (aka BGP NH tracking) • To scale IS-IS, operators would like to summarize PE loopbacks • However summarizing hides detailed reachability, BGP convergence then depends on BGP withdrawal

  6. Reachability and Routing • Currently IS-IS makes no distinction between having a route and having reachability • We call a route to an IP prefix “IP reachability” • As we move toward sophisticated control planes and highly efficient forwarding planes, a tension has developed

  7. Changing Economics and Priorities • Early 90s • Expensive memory • Slow processors • Applications tolerant of slow convergence • Summarization considered good for dataplane and control plane • Today • Memory much cheaper • Processors much faster • Applications demand fast convergence • For convergence fewer routes means faster convergence in the dataplane (primarily FIB update time) • But BGP next-hop tracking needs the reachability information • Needs of the control and data planes have diverged!

  8. Separating Routing and Reachability • New routing advertisement - SRDR • Summarized route • Detailed reachability • Straw-man format • Use the Extended IP Reachability TLV • Add a sub-TLV • Bit vector of reachable hosts Vector length = 2^(number of ignored bits)

  9. Example • Area 2 has 10.10.2.0/25 assigned as its address range • The following addresses appear in ABR2’s database for Area 2 10.10.2.1 - 10.10.2.27 10.10.2.46 10.10.2.74 - 10.10.2.87 • then the bit mask encoding would advertise a summary route to 10.10.2.0/25 with an associated 128-bit mask like this: 0 1 2 3 01234567890123456789012345678901 -------------------------------- 01111111111111111111111111110000 00000000000000100000000000000000 00000000001111111111111100000000 00000000000000000000000000000000

  10. Bit-Vector Characteristics • Limited to 1024 bits by TLV/sub-TLV encoding • Fixed size • Good for memory mgmt • Good for LSP fragmentation issues • Cannot exceed allowable sub-TLV size • Not compact for sparse allocation • Works well for IPv4 given the assumptions in the following case study

  11. Bit-Vector Case Study • Assume up to 20k routers in network • Break this into 50 domains • Average of 400 routers / domain • Assume PE are numbered in blocks of /24 addresses • Utilized 33% due to admin inefficiency • Requires 5 /24 per domain = 250 total • Each /24 would need 32 bytes of bit-vector • ~ 8k bytes total

  12. Detailed Reachabilty Encoding • These assumptions should carry over to IPv6 if provides allocate loopbacks from /120 addresses • Authors would like feedback on the assumptions from Service Providers

  13. Open Issues • Encoding • Encoding scheme not cast in concrete • One variation would be to have a new TLV instead of a sub-TLV of the Extended IP reachability TLV which would eliminate the /22 limitation on summarized addresses • Inconsistent advertisements

  14. Inconsistent Advertisements L1 10.10.2.2 PE2 ABR1 ABR3 ABR4 ABR2 PE1 L2 Domain L1 10.10.1.1 • How do ABR1, ABR2 react to inconsistent advertisements from ABR3, AB4? • How does PE1 react to inconsistent advertisements from ABR1, ABR2 • If no ECMP, then just relay the selected route’s information

  15. Inconsistent Advertisements • “Should” only happen in two cases • Race condition between L1L2 routers seeing a host/router come up or down • Area partition • Authors discussed several options • Leaking /32 of addresses one L1L2 router advertises vs the other L1L2 routers • Partition repair via L2 tunnel • Analysis is not complete at this time

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