BGP: Difference between revisions
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* BGP is an advanced path vector protocol and has following advantages: |
* BGP is an advanced path vector protocol and has following advantages: |
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Reliable updates |
Reliable updates (using TCP) |
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Triggered updates only |
Triggered updates only |
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Rich metric (Path attributes) |
Rich metric (Path attributes) |
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Scalable to massive networks |
Scalable to massive networks |
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Updates are Incremental and Triggered |
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Revision as of 18:10, 13 January 2019
Introduction
BGP is needed for redundancy of servers. BGP is not used for providing redundancy to users for internet access.
- Session establishment facts
BGP neighbors are not discovered. They must be configured manually on both sides of the connection. TCP port 179 is used. Only one session remains if both connection attempts succeed.
- BGP is an advanced path vector protocol and has following advantages:
Reliable updates (using TCP) Triggered updates only Rich metric (Path attributes) Scalable to massive networks Updates are Incremental and Triggered
BGP States
Idle Neighbor is not responding Active Attempting to connect Connect TCP session established OpenSent Open message sent OpenConfirm Response received Established Adjacency established
BGP Messages
Open Update Keepalive Sent every 60 seconds Notification Always indicate something is wrong
Protocol Specifications
Protocol Type | Path vector |
Peering mechanism | Manual peering between neighbors |
eBGP AD | 20 |
iBGP AD | 200 |
Rights | Open standard |
Supported protocols | IPv4, IPv6 |
Transport | TCP/179 |
Update mode | Only triggered |
Timers | Hello (60 sec) |
Authentication | None, MD5 |
Specifications | RFC 4271 |
- Usage applications
Customer connected to multiple Internet service providers (ISPs). Service provider networks (Transit autonomous system). Network cores of very large enterprise networks (distribution or core layer)as a backup or redundant routing protocol due to its stability.
BGP Route selection criteria[1]
Mnemonics: N-WLLA-OMNI
Full Internet BGP routing table is more than 300K routes and a BGP router can receive multiple copies of that routing table from multiple providers, router has to compare those multiple entries and select only the best route for the routing table. It uses the BGP Best Path Selection Algorithm to do this. Routes installed by different BGP instances are compared by the general algorithm, i.e. route distances are compared and the route with lower distance is preferred.
Attribute | Which is better |
---|---|
Next Hop reachable | Route cannot be used if next hop is unreachable |
Weight | Bigger |
Local Preference | Bigger |
Locally Injected | Locally injected is better than iBGP/eBGP learned |
AS Path Length | Smaller |
Origin | Prefer IGP over EGP & EGP over Unknown |
MED | Smaller |
Neighbor Type | Prefer eBGP over iBGP |
IGP Metric to Next Hop | Smaller |
Filter with Route Maps
Route maps are very powerful filtering tools, they can be used to accomplish the following tasks:
Filter on IP prefixes coming from a specific autonomous system Filter on other BGP attributes Modify BGP attributes
Match clauses in the BGP route map can be based on the following:
IP network numbers and subnet masks (prefix list or access list) Route originator Next hop Origin code Tag value attached to an Interior Gateway Protocol (IGP) route Autonomous system path Community IGP route type
With a route map, the following can be set:
Origin Next hop Weight Community Local preference MED
You can apply a route map on incoming or outgoing routing information for a neighbor. The routing information must be permitted by the route map to be accepted. If the route map has no statement explicitly permitting a route, the route is implicitly denied and dropped.
The syntax required is as follows:
Router(config-router)# neighbor ip-address route-map name in|out
What is a Route Reflector?[2]
This section is under construction. |
Synchronization
This section is under construction. |
Auto-Summarization
This section is under construction. |
MED vs Local Preference
This section is under construction. |
- The MED is an optional attribute that comes in handy when there are multiple entrance paths to an AS.
- The remote AS sets MED values to tell the other AS which path to use.
- The MED is passed between the two autonomous systems, but the value is not passed to any other ASes.
- The path with the lowest MED is the preferred path.
- This attribute is only used to influence entry INTO the AS.
- LOCAL_PREF is a well-known attribute that is also used when multiple paths between autonomous systems exist.
- The LOCAL_PREF attribute is just that… local and exclusive to the AS.
- Routers within the local AS are told what path to use to exit that AS.
- The local preference value is passed only among iBGP peers, and this value never leaves the local AS.
iBGP vs eBGP
This section is under construction. |
1. EBGP is peering between two different AS, whereas IBGP is between same AS (Autonomous System). 2. Routes learned from eBGP peer will be advertised to other peers (BGP or IBGP); however, routes learned from IBGP peer will not be advertised to other IBGP peers. 3. By default, EBGP peers are set with TTL = 1, which means neighbors are assumed to be directly connected, which is not in the case of IBGP. We can change this behavior for EBGP by using command “neighbor x.x.x.x ebgp-multihop <TTL>”. Multihop is the term used in EBGP only. 4. EBGP routes have administrative distance of 20, whereas IBGP has 200. 5. Next hop remains unchanged when route is advertised to IBGP peer; however, it is changed when it is advertised to EBGP peer by default. This default behavior of IBGP can be changed by the command “neighbor x.x.x.x next-hop-self”; this changes the next hop, while advertising, as a local route.
For IBGP peers dont need to be directly connected. Next-hop IP will not be changed when adv prefixes to another IBGP. Also, if you check the way BGP choose the best route to put in the routing table you'll see that EBGP is preferred. And BGP can run in two modes that each has a very different behavior when advertising routing information.
EBGP: external BGP runs between routers in different ASs. IBGP: internal BGP runs between routers in the same AS.
EBGP: routes received from an EBGP peer can be advertised to EBGP and IBGP peers. IBGP: routes received from an IBGP peer cannot be advertised to another IBGP peer but can be advertised to an EBGP peer.
Troubleshooting and Monitoring
Command | Description |
---|---|
show ip bgp neighbor ip-address | Displays detailed neighbor information |
show ip bgp | Displays all the routes in the BGP table |
show ip bgp ip-prefix [mask subnet-mask] | Displays detailed information about all paths for a single prefix |
debug ip tcp transactions | Displays all TCP transactions |
debug ip bgp events | Displays significant BGP events |
debug ip bgp keepalives | Debugs BGP keepalive packets |
debug ip bgp updates | Displays all incoming or outgoing BGP updates |
debug ip bgp updates acl | Displays all incoming and sent updates matching an ACL |
debug up bgp ip-address update [acl] | Displays all BGP updates received from or sent to a specific neighbor |
- BGP route not installing, route reasons:
This section needs verification or testing! |
Synchronization is enabled & route knowing by IGP Not Sync Next Hop inaccessible AS path includes the local AS Rejection by inbound policy
R&S Quick Notes
When using Communities, don’t forget “neighbor send-community” Know your attributes and the direction which applied, when to used what. “aggregate address” needs a more specific prefix in the BGP table for aggregate to be advertised. Synchronization issue has 3 solutions, 1- Load BGP on all transit routers, 2- GRE tunnel, 3- Redistribution BGP>IGP. “no bgp nexthop trigger” – Disables next-hop tracking between scanner intervals. “no bgp fast-ext-fallover” – Force the router to wait for the dead-timer to expire, before generating notification messages , when a connected peer goes down. “neighbor fall-over” – Will check neighbor connenctivity between scanner intervals, aka BGP Fast Peering. Only the Holdtime is sent in update-msg. Two neighbors will use the lowest holdtime and then calculate the keepalive from that. Know your Regular Expressions Know the difference between Peer-Groups and Peer-Templates
LAB
- Complete BGP Lab:
This section is under construction. |
References
{{#widget:DISQUS |id=networkm |uniqid=BGP |url=https://aman.awiki.org/wiki/BGP }}