DD2491 p Inter-domain routing and BGP BGP in JunOS. Olof Hagsand KTH/CSC

Transcription

1 DD2491 p Inter-domain routing and BGP BGP in JunOS Olof Hagsand KTH/CSC

2 JunOS Routing model Neighbour s Neighbour s Import Protocols RIB Export Protocols FIB Note: Export policies may be applied only to active routes! Protocol Default import action Default export action direct and static accept all N/A RIP accept all RIP routes reject all BGP accept all BGP routes export all active BGP routes IS-IS accept all IS-IS routes reject all (IS-IS uses LSAs) OSPF accept all OSPF routes reject all (OSPF uses LSAs) MPLS accept all MPLS routes export all active MPLS routes

3 BGP Routing Process Model Peer Peer Peer import policy decision process RIB export policy Pool of routes received from peers Import policy for filtering and attribute manipulation Decision process to select best routes Pool of routes used by router Export policy for filtering and attribute manipulation Pool of routes that the router advertises Peer

4 BGP Routing Information Bases (BGP RIBs) CISCO version Input Policy Engine Output Policy Engine Adj-RIB-In Adj-RIB-Out Adj-RIB-In Adj-RIB-Out BGP decision process Loc-RIB Adj-RIB-In Adj-RIB-Out Adj-RIB-In Adj-RIB-Out 2001 Cisco Press

5 BGP RIBs BGP routing table consists of three parts Adj-RIB-In One per peer BGP speaker Stores routing information learned from peer Filtered/manipulated input policy engine Loc-RIB Selected best routes by decision process to each available destination Adj-RIB-Out One per peer BGP speaker Stores routing information selected for advertisement to peer Output policy applied to Loc-RIB before going into Adj-RIBOut This is redistributed if REFRESH capability is used

6 Import/Export Policy Import policy Affects routes received from peer BGP speakers Filtering based on IP prefixes, AS_PATH and other BGP attributes Manipulates path attributes to influence its own decision process Export policy Affects routes in Loc-RIB (candidates for advertisement) In JunoS: only active BGP routes Differentiates between internal and external peers

7 BGP example policies AS /24 0/0 import policy AS / /24 0/0 Use /24 from AS1 Use 0/0 and /24 from AS /24 AS3 decision process RIB export policy / /24 Deny 0/0 from AS1 Give /24 from AS1 better pref Do not propagate 0/0 Do not announce /24 to AS3 Give /24 metric 10 toward AS4 0/ / /24 AS4 AS2 BGP AS1 BGP AS2 BGP

8 Configuring BGP in JunOS protocol bgp { mtu-discovery group external-peers { type external; peer-as 42; neighbor ; neighbor ; neighbor { peer-as 93; Global properties Group properties Peer properties Many configurations can be made on global, group and peer level. More specific is preferred (peer before group before global) See:

9 BGP commands in JunOS (1) advertise-inactive advertise-peer-as + > > > + Advertise inactive routes Advertise routes received from the same autonomous system authentication-algorithm Authentication algorithm name authentication-key MD5 authentication key authentication-key-chain Key chain name cluster Cluster identifier damping Enable route flap damping description Text description disable Disable BGP export Export policy family Protocol family for NLRIs in updates graceful-restart BGP graceful restart options group Define a peer group hold-time Hold time used when negotiating with a peer import Import policy include-mp-next-hop Include NEXT-HOP attribute in multiprotocol updates ipsec-sa IPSec SA name keep How to retain routes in the routing table

10 BGP commands in JunOS (2) local-address > local-as local-preference log-updown > metric-out mtu-discovery > multihop no-advertise-peer-as no-aggregator-id out-delay passive > path-selection peer-as preference remove-private tcp-mss > traceoptions Address of local end of BGP session Local autonomous system number Value of LOCAL_PREF path attribute Log a message for peer state transitions Route metric sent in MED Enable TCP path MTU discovery Configure an EBGP multihop session Don't advertise routes received from the same autonomous system Set router ID in aggregator path attribute to 0 How long before exporting routes from routing table Do not send open messages to a peer Configure path selection strategy Peer autonomous system number ( ) Preference value Remove well-known private AS numbers Maximum TCP segment size ( ) Trace options for BGP

11 Routing policy: syntax and flow Changing the default routing policy Syntax: policy-options { policy-statement name { term term-name { from { match; then { action; route rejec t Policy 1 term1 term2 term3 Policy 2 term1 term2 term3 default policy term acce pt next

12 Applying policies protocol bgp { export p0; import p1; group external-peers { type external; export p2; import p3; neighbor { export p4; import p5; Global properties Group properties Peer properties Export policy evaluation order: p4->p2->p0 If verdict (accept, reject) policy chain is terminated Side-effects may still apply

13 Policy-options statements # set policy-options? as-path name reg-exp Create a named AS-PATH regular expression Example: as-path asp {4 as-path-group { [as-path] community name members [ ids ] Example: community c0 members 701:555 damping name [options] policy-statement prefix-list name { ip-addresses Create a named list of prefixes Example: prefix-list p0 { ; /24;

14 Policy-statement matches # set policy-options policy-statement <name> term <name> from? as-path community family local-preference metric neighbor next-hop origin preference prefix-list protocol route-filter...

15 Policy-statement actions # set policy-options policy-statement <name> term <name> then? accept reject next policy next term Side-effects with accept: trace as-path-prepend community color external load-balance per-packet local-preference metric next-hop origin preference

16 Lab overview Tier1 AS /27 RTX1.2 0/0/0 1/0/0 1/0/ /0/0 1/0/1 1/0/0 RTX2 RTX /0/0 1/0/1 1/0/0 RTX3 1/0/1 AS6500X-1 1/0/0 AS6500X 10.X.8.0/24 10.X.9.0/24 10.X.10.0/24 10.X.11.0/24 AS6500X+1