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In BGP (Border Gateway Protocol), managing route updates and reapplying new policies can sometimes be challenging, especially if you want to avoid resetting the BGP session. Two methods allow you to update routing policies without tearing down the session: BGP Soft Reconfiguration and BGP Route Refresh . While both methods serve the same purpose, they work differently and have distinct impacts on your router's resources. This post explains the key differences between Soft Reconfiguration and Route Refresh , when to use each, and why Route Refresh is preferred in most modern networks. 1. What is BGP Soft Reconfiguration? BGP Soft Reconfiguration is an older method of applying new policies (like route maps, filters, or prefix lists) without resetting the BGP session. It works by storing a local copy of all the routes received from a BGP neighbor before applying inbound policies. This local route copy allows the router to reprocess the routes when a policy change occurs. How So...

AS Path Prepending is a BGP feature used to make a specific path appear less preferred by artificially lengthening the AS path. This is done by adding your AS number multiple times to the AS path. It is a common method to influence inbound traffic from external networks. Longer AS Path = Less preferred route . Example Scenario : You have two ISPs: ISP1 (through CE1) and ISP2 (through CE2). You want inbound traffic from the internet to prefer ISP1 over ISP2. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP2): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE2 (AS Path Prepending to Make ISP2 Less Preferred) : Create a route map to prepend your AS path multiple times for CE2: route-map PREPEND_AS permit 10 set as-path prepend 65001 65001 65001 Apply this route map to the neighbor in the BGP configuration for CE2: router bgp 65001 neighbor 10.0.2.1 remote-as 65002 neighbor 10.0.2.1 ro...

The Multi-Exit Discriminator (MED) is used in BGP to control inbound traffic into your AS. It tells a neighboring AS which entry point into your network it should prefer when there are multiple links between your AS and the neighboring AS. The lower the MED value , the more preferred the path. MED is only honored between the same neighboring AS . Example Scenario : You are connected to ISP1 via two routers, CE1 and CE2 , and want to control which router ISP1 uses to send traffic into your AS. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP1): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Lower MED, More Preferred) : Create a route map to set the MED to 50 for CE1: route-map SET_MED permit 10 set metric 50 Apply this route map to the neighbor in the BGP configuration for CE1: router bgp 65001 neighbor 10.0.1.1 remote-as 65000 neighbor 10.0.1.1 route-map SET_MED out Configuratio...

In BGP, Local Preference is used to control the outbound traffic path. It helps you decide which egress point (exit point) should be used when you have multiple connections to external networks, such as ISPs. Local Preference is an attribute that is local to your AS and is shared with all iBGP peers but not with eBGP neighbors. Higher Local Preference = More preferred outbound path. Example Scenario : You have two external links: ISP1 (via CE1) and ISP2 (via CE2). You want traffic to prefer ISP1 for all outbound traffic. Network Topology : CE1 (connected to ISP1): 10.0.1.1/30 CE2 (connected to ISP2): 10.0.2.1/30 iBGP Router (Internal) connected to both CE1 (10.0.1.2/30) and CE2 (10.0.2.2/30). Configuration on CE1 (Higher Local Preference) : Create a route map to set the local preference to 200 for routes learned from CE1: route-map SET_LOCAL_PREF permit 10 set local-preference 200 In the BGP configuration for CE1, apply this route map to the neighbor: router bgp 65001 ne...

In this lab, we’ve set up a basic MPLS, BGP, and L3VPN environment, which is a great foundation for understanding how service providers build scalable networks. The lab uses the EVE-NG simulator along with Router IOS C7200-ADVENTERPRISEK9-M, Version 15.2(4)M8 to emulate a realistic MPLS environment. Below is a summary of the key components and roles of each router in the lab. MPLS Core Routers : The MPLS core consists of the routers responsible for label switching and forwarding customer traffic through the network: PE1 (Provider Edge 1) : Connects customer networks to the MPLS core and handles both MPLS and BGP routing. It also hosts VRF (Virtual Routing and Forwarding) instances for customers. PE2 (Provider Edge 2) : Functions similarly to PE1, connecting another customer network to the MPLS core. P1 (Core Router 1) and P2 (Core Router 2) : These routers serve as MPLS core routers and handle label switching but do not store or process customer routes directly. They simply f...

Here’s a breakdown of BGP attributes that are either considered by iBGP neighbors only or eBGP neighbors only , along with the attributes that apply to both, but may have different behaviors or implications depending on whether the neighbor is iBGP or eBGP. Attributes Considered by iBGP Neighbors Only : These attributes are shared within an AS but may not be propagated or considered by eBGP neighbors : Local Preference : Used by : iBGP Ignored by : eBGP Description : The Local Preference (Local Pref) attribute is used to influence outbound traffic within an AS. It is not sent to eBGP neighbors . An eBGP neighbor won’t see this attribute because it’s meant for internal path selection. Example : An iBGP router receiving an update with a higher Local Preference will prefer that path, but an eBGP neighbor will not receive or consider the Local Preference attribute. Next-Hop Behavior : Used by : iBGP Modified by : eBGP Description : When advertising routes to iBGP neighbors, the Next ...

 AFI (Address Family Identifier): AFI is a field in BGP that identifies the network layer protocol for which BGP is advertising routes. It specifies the type of addresses being advertised, such as IPv4 or IPv6. Examples of AFI values : 1 for IPv4 2 for IPv6 SAFI (Subsequent Address Family Identifier): SAFI is a field in BGP that provides more specific information about the type of NLRI (Network Layer Reachability Information) being advertised within the AFI. It defines how the addresses within the AFI should be treated, such as unicast, multicast, or VPN routes. Examples of SAFI values : 1 for Unicast 2 for Multicast 128 for MPLS-labeled VPN (VPNv4) In Brief: AFI tells which address family is being used (e.g., IPv4 or IPv6). SAFI tells how the routes in that address family should be interpreted (e.g., unicast, multicast, or VPN).

Separation of Address Families in BGP : In modern versions of Cisco IOS, BGP is designed to support multiple address families beyond just IPv4. BGP can handle: IPv4 unicast (standard routing for IPv4 addresses) IPv6 unicast (for IPv6 routing) VPNv4 and VPNv6 (for MPLS Layer 3 VPNs) Multicast for IPv4 or IPv6 And other extensions like EVPN or MPLS VPN . The address-family command is used to tell BGP which specific type of routes you want to activate. By default, no address-family is active, so you need to manually specify which ones BGP should work with. Default BGP Behavior : In older Cisco IOS versions, BGP only supported IPv4 unicast by default, so this wasn't an issue. However, newer versions of IOS require explicit activation of the IPv4 unicast address family to avoid ambiguity and to support flexibility for other address families. Without the address-family ipv4 activation, even though you configure neighbors in BGP, no routes would be exchanged because BGP doesn...